In the applicant's Norwegian application no. 950916 there is disclosed a device of the above-mentioned type, wherein the compressor device is composed of a turbocompressor and the pressure of the air that is supplied to the free piston device's combustion chamber during an air purging sequence substantially corresponds to the compressor device's supply pressure. The energy for operating the compressor can be provided, e.g., by an electric motor that is supplied with electric power from a generator, which in turn is driven by the turbine.